main.c: always #define VERSION
main.c: always #define VERSION

This makes compilation easier without using the
Makefile. `gcc *.c` would fail like this:

$ gcc *.c
main.c: In function ‘main’:
main.c:63:32: error: expected ‘)’ before ‘VERSION’
   63 |     fprintf(stderr, "earlyoom " VERSION "\n");
      |                                ^~~~~~~~
      |                                )

Now it works.

rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom